I just found an unusual issue, which turned out to not be an IntelliJ problem.
Every time I switched back from another application to IntelliJ, there would be a noticeable delay that locked up the application. Up to 10 seconds sometimes. Once the delay was finished, the application performed just fine.
Obviously, this seems like a Java or memory issue, but that isn't the case on my computer. However, I did have a Mac utility called MenuAnywhere running, which puts a second menu on my other monitor. (There are other applications like this.) It works by scanning through the menus and building a virtual menu based on the main one, using accessibility hooks.
Simply put, disabling this program completely eliminated the issue. It might have even sped up the application in general.
So, if you are experiencing unusual slowdowns, and JVM adjustments aren't helping, see if you have any utilities that are scanning the menus.